1. Field of the Invention
The present invention relates to a surface acoustic wave device, especially to a surface acoustic wave device used as high-frequency filters or resonators and to a method of producing the same.
2. Description of the Related Art
Advancements in mobile communication technologies in recent years have been causing communication equipment to become much more compact and operate at higher frequencies. Such equipment requires oscillators and high-frequency filters as indispensable components which often comprise surface acoustic wave devices.
Conventional surface acoustic wave devices, such as surface acoustic wave filters and surface acoustic wave resonators, are made by forming interdigital transducers on a piezoelectric substrate such as lithium niobate or lithium tantalate, whereon the surface acoustic wave is generated by applying an alternate electric field to the interdigital transducers. A surface acoustic wave device used in the mobile communication equipment must have good operation characteristics in the high frequency range. High-frequency characteristics of a surface acoustic wave is evaluated as a frequency pass-band and an insertion loss in the case of a filter, and a resonation Q value which corresponds to the inverse of loss in the case of a resonator. The frequency pass-band is determined by the electromechanical coupling factor and the temperature dependence thereof of the piezoelectric material to be used, which generally have values characteristic of the piezoelectric material used. Thus the possible magnitude of the frequency pass-band is roughly determined by what piezoelectric material is used. In the case of lithium niobate having a relatively high electromechanical coupling factor and low dependence on the temperature, for example, the electromechanical coupling factor is about 5 to 10% and the temperature dependence is 70 to 100 ppm/.degree.C. Temperature dependence is desired to be as low as possible.
The vibration frequency of a surface acoustic wave device is inversely proportional to the interval and width of the interdigital transducers. In order to make a filter or a resonator having a vibration frequency of 1 GHz, for example, the width and interval of the strip line of the interdigital transducers should be about 1 .mu.m when lithium niobate or lithium tantalate is used. Because the mass of the electrode has an adverse effect on the oscillation characteristics of the surface acoustic wave, it is required to reduce the electrode mass as the dimension of the interdigital transducer decreases. In actual applications, a light metal such as aluminum is used with the thickness being set to within 0.1 .mu.m in order to fabricate the interdigital transducer. As a result, resistive loss due to the electric resistance of the interdigital transducers increases as the frequency increases. Consequently, increase in the frequency leads to an increase in the insertion loss in the case of filter, and to a decrease of Q value in the case of resonator.
When a surface acoustic wave device is used in a radio transmitter section, power capacity becomes a critical problem. In the case of mobile telephone, power capacity of about 0.2 watts is sufficient in the receiver section, though the transmitter section requires a power capacity of about 2 watts at the front end thereof. Because the interdigital transducers of a surface acoustic wave device are fastened directly on the oscillating section of the surface acoustic wave, the electrodes are constantly exposed to mechanical vibration and therefore may be physically destroyed (breakage due to stress migration) when they are subject to vibration having a large magnitude of power. When the interdigital transducers are made in small dimensions for operation at higher frequencies, in particular, it easily fails due to line breakage or other causes. Thus it has not been able to obtain a surface acoustic wave device having high power capacity.
There also have been problems in the aspects of manufacture and application, in that the smaller distance between interdigital transducers makes insulation failure between interdigital transducers easier to occur due to the pyroelectric effect.
As described above, conventional surface acoustic wave devices have such problems as increasing insertion loss, decreasing Q value of resonance, inability to obtain high power capacity and susceptibility to pyroelectric breakage, as the interdigital transducer is made in smaller dimensions for the operation at higher frequencies.
One known constitution of surface acoustic wave device of the prior art is that proposed in the Japanese Patent Publication No. 3-6912 wherein interdigital transducers, being arranged above a piezoelectric substrate with an air gap secured in-between, are supported by an insulator bridge made of a non-piezoelectric material. More specifically, the surface of the piezoelectric substrate is covered with a polysilicon film 50 to 150 nm thick whereon interdigital transducers made of Au 2 .mu.m thick is formed, with a SiO.sub.2 insulator film 5 .mu.m thick covering thereon, then an opening for the polysilicon is made through which the polysilicon is selectively removed by etching and interdigital transducers are made to oppose the piezoelectric substrate via an air gap of 50 to 150 nm. Because the interdigital transducers are placed via an air gap, the mass of the interdigital transducers does not affect the oscillating section of the surface acoustic wave. Thus it is made possible to reduce both the spurious and the insertion loss.
In the constitution disclosed in the above patent, however, because the insulating support holding the interdigital transducers is a thin film formed by vacuum evaporation or other means, its mechanical strength is not sufficient and therefore does not have sufficient mechanical reliability against vibration required for such applications as mobile communication equipment. Specifically, there are serious practical problems such as breakage of the electrode in vibration test or drop test and change in the air gap causing the electromechanical coupling characteristic, thereby leading to a deterioration of the performance. Because the insulating support must be made of a material which can be formed by the thin film technology, applicable materials are limited and it is difficult to obtain a suitable material having appropriate thermal expansion coefficient. No consideration is given to the temperature dependence. The above construction has another problem of greater device size because it requires to be housed in a package for hermetic seal.
The present invention overcomes aforementioned shortcomings associated with conventional surface acoustic wave devices and provides a compact surface acoustic wave device capable of providing good high-frequency characteristics for filter and resonator, in particular low loss, high Q value of resonance, high power capacity, good temperature characteristic, high mechanical reliability and good pyroelectric resistance even at high frequencies.